|This 1/2" valve has been the been one of the main flow valves used by robbens over the years. It is usally found as the main flow valve connected to the individual loops of pipe going into the floor as the heating circuits. They are usually easy to replace either the entire valve or just the inserts which are available as the part VAP0036. The previous valve to this had a plastic insert which cannot be replaced. In which scenario you must repalce the entire valve.
Back around the year 2000 they were soldered onto the manifolds and can usually be recognised by being painted black along with the main manifold rails. In this situaion it would be easier to replace the insert only as removal of the valve in situ can be very difficult and would be best done on a bench in a workshop with the correct tools.